Styrene and related vinyl aromatic compounds are the building blocks for numerous types of industrial products such as, for example, polymers and resins. Styrene can be produced as a commodity chemical through recovery from a hydrocarbon stream (e.g., pyrolysis gasoline) or by dehydrogenation of ethylbenzene. Isolation of a vinyl aromatic compound (e.g., styrene) from a pyrolysis gasoline stream is typically performed by extractive distillation, which typically necessitates exposure of the vinyl aromatic compound to elevated temperatures and/or oxygen. Both of these conditions can initiate unwanted thermal- or free radical-induced polymerization or oligomerization. Polymerization can lead to production losses and eventually result in system blockages of the apparatus being used for separating the vinyl aromatic compound from the hydrocarbon stream.
To lessen polymerization of vinyl aromatic compounds during their isolation from a hydrocarbon stream, extractive distillation is conventionally performed at reduced pressures to minimize oxygen exposure and lower processing temperatures to minimize thermal exposure. Even under the best of circumstances, however, the vinyl aromatic compounds will still come into contact with small quantities of oxygen and be exposed to temperatures ranging from about 80° C. to about 160° C. for time periods ranging from seconds to hours. Thermal-induced polymerization of styrene can occur when the styrene monomer is exposed to temperatures of about 100° C. or higher for only a few minutes. In addition to the aforesaid oxygenation and thermal conditions, the vinyl aromatic compounds may also be exposed to a variety of contaminants in the hydrocarbon source such as, for example, sulfur-containing compounds and colored impurities, that may further promote unwanted polymerization.
Methods are known in the art for minimizing the polymerization of vinyl aromatic compounds, particularly styrene, through adding a small amount of an inhibitor compound to the vinyl aromatic compound in either purified or crude form. For example, dinitrophenolic compounds (e.g., 2,6-dinitro-p-cresol) have been used to inhibit polymerization of vinyl aromatic compounds during vacuum distillation. A combination of a hindered phenol (e.g., a dinitrophenolic compound), optionally a hydroxylamine, and a phenylenediamine have also been used to inhibit vinyl aromatic compound polymerization under oxygen-free conditions or under normal atmospheric conditions. Likewise, a combination of dinitrophenolic compound and a nitroxyl free radical compound of have been used to inhibit vinyl aromatic compound polymerization. A combination of a 2-nitrophenolic compound in combination with a sulfonic acid compound has also been used to inhibit vinyl aromatic compound polymerization.
Although the aforesaid inhibitor systems are generally effective for production and purification of styrene via conventional dehydrogenation of ethylbenzene, Applicants believe that they are unsuitable for extractive distillation of styrene and other vinyl aromatic compounds from a hydrocarbon source such as, for example, pyrolysis gasoline. In view of the foregoing, new methods for inhibiting the polymerization of vinyl aromatic compounds during their isolation from a hydrocarbon stream would generally be beneficial in the art. Recent advances in separations technology make it possible to recover styrene from pyrolysis gasoline derived from the steam cracking of naphtha, gas oils and liquid natural gas. These new separations technologies include conditions of thermal exposure and oxygen exposure that are generally challenging parameters for maintaining stability of styrene and other vinyl aromatic compounds in an unpolymerized state. Embodiments set forth herein are effective in overcoming those challenges.